1. Field of the Invention
The present invention relates to furnaces for reacting a gas capable of an exothermic reaction phase followed by an endothermic reaction phase. More particularly, the present invention relates to a furnace for producing a protective atmosphere of 40% N.sub.2, 40% H.sub.2, and 20% CO (nominally).
2. Description of the Prior Art
Numerous industrial operations require a protective or shielding atmosphere such as a 40% N.sub.2, 40% H.sub.2, and 20% CO (nominally) atmosphere. These industrial applications include the hardening of medium and high carbon steels, the annealing and normalizing of carbon steels, carburizing carbonitriding, copper and silver brazing and sintering.
It has been known that a protective atmosphere containing a nominally 40% N.sub.2, 40% H.sub.2, and 20% CO (with residual CO.sub.2, H.sub.2 O and CH.sub.4) could be produced by the cracking of a hydrocarbon gas over a catalyst bed at high temperature. The hydrocarbon gas could include such gases as natural gas, propane, butane, of MFG (manufactured gas). Further, furnaces or generators have been known for carrying out such a cracking process. One such furnace is the HYEN endothermic generator manufactured by the Lindberg Company of Chicago, Ill.
The HYEN endothermic generator is illustrated in FIG. 1. It consists of an elongated tubular metallic housing 2 having a length of from approximately 45 to approximately 69 inches and a diameter of approximately 8 inches. The tube has a closed bottom 4 including a reaction gas inlet 6 and insulation layer 8. The top of the tube is also closed except for reacted gas outlet 10. A reacted gas passage 12 adjacent the top of the tube includes insulation 14.
Screen 16 is supported by the insulation 8 and prevents the catalyst from plugging the gas inlet 6. Over the screen is placed material having good heat transfer characteristics, such as alundum chips 18, while the remainder of the tube is filled with catalyst material. The catalyst material is in the form of porous 1/2" to 1" cubes or spheres impregnated with nickel oxide.
The furnace is normally located within an insulating jacket (not shown). Heat may be supplied to the furnace by either surrounding the furnace with electric resistance coils (not shown) or by supplying hot combustion gases to the interior of the insulating jacket.
In use, a reaction gas, such as natural gas, is mixed with air in a carburetor and the mixture enters the furnace at the bottom inlet pipe 6. The catalytic cracking of the hydrocarbon gas in the mixture is a two phase reaction; the reaction has a first exothermic phase followed by a subsequent endothermic phase. As the reaction gas mixture enters into the alundum chip bed, it is heated by the heat source until the gas mixture reaches minimum temperature for exothermic combustion to commence (approximately 1170.degree. F.). The heat liberated during the exothermic phase is absorbed by the gas mixture raising its temperature into the endothermic range which commences at approximately 1750.degree. F. During the endothermic phase, heat from the heat source is required to maintain the reaction gas temperature above 1750.degree. F. until the cracking process is completed.
The insulation 14 in the reacted gas passage prevents premature cooling of the reacted gas and the reacted gas product exits at 10 at a temperature of approximately 1700.degree..
For example, a mixture of one part natural gas to 2.44 parts air was preheated to 200.degree. F., introduced into the HYEN generator and reacted at a rate of 1513 cubic feet per hour (CFH). Heating power was supplied to the generator at the rate of 17.77 kilowatts per hour (KW/HR). The reacted gas exited the generator at 1700.degree. F. with a dew point of 28.8.degree. F., a methane content of 0.46%, a carbon dioxide content of 0.65%, a carbon monoxide content of approximately 20%, a hydrogen content of approximately 40%, and a nitrogen content of 40%.
However, the HYEN endothermic generator described above has several shortcomings. First, it fails to provide a uniformity of gas heating during the reaction process. When the gas first enters the bottom of the catalyst bed 20 this area has been heated by a combination of heat supplied by the heat source and exothermic reaction taking place in the alundum chip bed. As the reaction gas travels up through the catalyst bed, all additional heat needed to continue the endothermic reaction must be supplied by the heat source.
Since this reaction is initially exothermic, heat is created and driven into the center portion of the 8" diameter furnace retort forcing complete exothermic reaction throughout the first approximately 12" height of the alundum chip bed and the lower portion of the catalyst bed within the retort. Factually, this area is not entirely exothermic as the dissociation of H.sub.2 O (water vapor) and CO.sub.2 (carbon dioxide) are endothermic and also occur in this area simultaneously.
Further, because of the relatively large 8" diameter of the generator retort 2 and the insulating quality of the catalyst bed therein, there is insufficient heat penetration into the center portion of the catalyst bed.
Therefore, a "cold spot" is formed in the center of the generator retort where the temperature is below the desired reaction range, and the gas moving through this cold spot is deprived of heat during the final portion of the travel through the generator retort. As a result, in order to provide sufficient heat so that all of the gas is reacted, it is necessary to raise the heat source temperature gas to approximately 200.degree. to 300.degree. F. higher than the minimum cracking temperature in order to assure that all reacting gas within the generator is above 1750.degree. F., the minimum cracking temperature. The reacted gas entering the insulated area 14 thus ranges in temperature from approximately 1750.degree. F. to 1800.degree. F. and exits the retort at approximately 1700.degree. F.
If a proper reaction does not take place due to insufficient heating to the central core of the catalyst bed, the residual methane (CH.sub.4) content can drop out as carbon soot by the reaction CH.sub.4 .fwdarw.C+2H.sub.2.
Further, excessive sooting can take place downstream of the reacted gas outlet 10 if cooling is too slow to "freeze" the reverse reaction (2 CO.fwdarw.C+CO.sub.2), thereby producing carbon if the temperature drops too slowly through the range of 1250.degree. to 900.degree. F.
Further, it is necessary to provide the reaction gas with a head temperature of 200.degree. F. to 300.degree. F. to assure that a proper reaction takes place which also requires excessive amounts of energy at the furnace heat source.
Further, the need for a 200.degree.-300.degree. F. head temperature requires the need for a higher grade of alloy retort and electric heating element material and furnace refractory insulation resulting in excessive material costs.
Further, since the reacted gas leaving the furnace at approximately 1700.degree. F. temperature enters immediately into a heat exchanger, following the furnace, in order to cool the gas to a useful processing temperature (approximately 150.degree. F.), all the heat within the reacted gas on leaving the catalyst bed, and removed by the heat exchanger (.DELTA.T 1700.degree. F..rarw..fwdarw.150.degree. F.), is thereby being wasted.
Finally, because of the high temperature involved, more expensive construction material must be used and the life of the material is shortened.